The present invention relates generally to the production of alkenyl-substituted heterocyclic compounds, and in particular to processes for preparing vinyl-substituted heterocycles such as 2- or 4-vinylpyridines by reaction of corresponding picolines and formaldehyde in the vapor phase over a solid catalyst.
As further background, alkenyl-substituted heterocyclic compounds such as 2- and 4-vinylpyridine are useful in the preparation of polymers which enjoy a wide variety of applications, including for example fabric stiffeners for the production of biased-ply car tires, in the preparation of ion exchange resins, in dye transfer inhibition, and in the research and medical fields generally. With the goal of developing facile syntheses, a few one-step routes to vinylpyridines have been proposed.
For instance, Bonnemann et al., have proposed a reaction of two moles of acetylene with acrylonitrile using a soluble [(xcex76-1-PhC5H5)Co(1,5-cyclooctadiene)]. Applied Homogeneous Catalysis with Organometallic Compounds, Cornils, B., Herrmann, W. A., Eds.; VCH: Weinheim, Germany, 1996, Volume 2, pp. 1106-1107.
A few reports also propose a one-step vapor-phase production of vinylpyridine over a solid catalyst. More recently, the oxidative dehydrogenation of 2-ethylpyridine to 2-vinylpyridine over SnO2/SiO2 (Moscotti et al., Applied Catalysis 1996, 134, 263-274) or molybdenum-based catalysts (Belomestnykh et al., Chemistry of Heterocyclic Compounds, 1994, 30, 701-708) have been proposed. Others have suggested the oxidative coupling of CH4 with xcex1-, xcex2-, and xcex3-picoline to give the corresponding vinylpyridine over Naxe2x80x94Cs/MgO (Ruckenstein, et al., Catalysis Letters, 1994, 29, 217-224). Watanabe et al., in an earlier report proposed the use of H3PO4/SiO2, and ZnX2/SiO2 (Xxe2x95x90Cl, F) to produce 2- and 4-vinylpyridine from picoline/formaldehyde feeds.
In light of this background, there remains a need for an improved process for preparing alkenyl-substituted heterocycles such as 2- and 4-vinylpyridines. The present invention addresses this need.
It has been discovered that alkenyl-substituted heterocycles can be prepared effectively by the reaction of formaldehyde with a corresponding alkyl-substituted heterocycle in the vapor phase in the presence of a zeolite catalyst, wherein the catalyst contains both acidic and basic catalytic sites. Accordingly, in one preferred aspect, the present invention provides a process for preparing a vinyl-substituted nitrogenous heterocycle, comprising reacting a methyl-substituted nitrogenous heterocycle with formaldehyde in the vapor phase in the presence of a zeolite catalyst modified with at least one metal cation that provides basic catalytic sites, so as to form a corresponding vinyl-substituted nitrogenous heterocycle. In a most preferred form, this process involves reacting an xcex1-picoline or xcex3-picoline with formaldehyde in the presence of a medium pore zeolite catalyst modified with at least one metal cation so as to form a corresponding 2-vinylpyridine or 4-vinylpyridine compound. The metal cation is preferably an alkali metal cation, more preferably selected from the group consisting of sodium, potassium, rubidium and cesium. Processes of the invention are effectively carried out at temperatures in the range of about 250xc2x0 C. to about 500xc2x0 C., more preferably about 350xc2x0 C. to about 450xc2x0 C. Advantageous processes are carried out with a relatively low ratio of picoline (or other starting heterocycles) to formaldehyde, for example less than about a 5:1 molar ratio, more preferably less than about a 5:1 molar ratio, more preferably less than about a 3:1 molar ratio. Most preferred processes are conducted with a starting heterocycle (e.g. picoline) to formaldehyde molar ratio of about 1:1 or less. Desirably, the zeolite employed has a constraint index of about 0.5 to about 12, and can be a phosphorous-stabilized zeolite.
The present invention provides novel processes for preparing vinyl-substituted heterocycles which can be conducted in the vapor phase as one-step reactions while providing high yields and selectivities and not requiring the use of high picoline-to-formaldehyde ratios. Additional objects and advantages of the invention will be apparent from the descriptions herein.
For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the certain embodiments thereof and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations, further modifications and applications of the principles of the invention as described herein being contemplated as would normally occur to one skilled in the art to which the invention relates.
As disclosed above, the present invention provides novel processes for the production of alkenyl-substituted heterocycles, for example vinyl-substituted heterocycles. The inventive processes are conducted in the vapor phase over zeolite catalysts. Processes of the invention involve the reaction of an appropriate alkyl-substituted nitrogenous heterocycle with formaldehyde at an elevated temperature, to yield a corresponding alkenyl-substituted nitrogenous heterocycle. More preferred processes of the invention include the reaction of xcex1-picoline or xcex3-picoline under such conditions to provide the corresponding 2-vinylpyridine or 4-vinylpyridine compound.
Turning now to a more detailed discussion of the starting materials for processes of the invention, the alkyl-substituted heterocycle is generally an aromatic heterocyclic compound containing one or more nitrogens in the ring, preferably a single nitrogen in the ring. The aromatic ring will typically contain from about 4 to about 6 members. Particularly preferred starting compounds are pyridines and pyrazines containing an alkyl group (e.g. a C1 to C5 alkyl group, especially methyl, ethyl or propyl) on an activated position of the ring, for example the 2- or 4-position of a pyridine ring as in xcex1-picoline, xcex3-picoline, or the 2-position of a pyrazine ring as in 2-methylpyrazine. The heterocycle may also have a substituent on one or more non-reactive positions of the heterocyclic ring which will not be altered under the conditions of the reaction, for example on the 3-position and/or 5-position of a pyridine ring. Such substituent may be, for example, a C1 to C5 hydrocarbon group such as an alkyl group. Suitable starting materials for the present invention thus also include 2,3-dialkypyridines, for example 2,3-dimethylpyridine and 2-ethyl-3-methylpyridine which may be reacted with formaldehyde to form 2-vinyl-3-methylpyridine and 2-propenyl-3-methylpyridine, respectively; 2,5-dialkypyridines, for example 2,5-dimethylpyridine and 2-ethyl-5-methylpyridine which may be reacted with formaldehyde to form 2-vinyl-5-methylpyridine and 2-propenyl-5-methylpyridine, respectively; and 2,3,5-trialkylpyridines, for instance 2,3,5-trimethylpyridine and 2-ethyl-3,5-dimethylpyridine which can be reacted with formaldehyde to form 2-vinyl-3,5-dimethylpyridine and 2-propenyl-3,5-dimethylpyridine, respectively.
The alkyl-substituted heterocycle will be reacted with a compound containing a single carbon, preferably formaldehyde. In this regard, the formaldehyde in the reaction zone can be provided by feeding formaldehyde itself to the reaction zone, or compounds which form formaldehyde under the conditions of the reaction. For example, the formaldehyde feed may take the form of an aqueous solution of formaldehyde, preferably a concentrated solution. Illustratively, an aqueous solution containing twenty weight percent or more formaldehyde, preferably thirty weight percent or more formaldehyde, can be used. Alternatively, the formaldehyde feed may take the form of trioxane, which essentially provides the equivalent of three formaldehyde molecules to the reaction zone.
Processes of the invention will employ a zeolite catalyst modified to contain both acidic and basic catalytic sites. In this regard, zeolites are known to contain native acidic (H+) sites. In accordance with the invention, the zeolite will be modified with an appropriate metal cation to provide basic catalytic sites as well. Suitable metal cations will be known to those skilled in the art, but preferably the metal cation is an alkali metal cation or an alkaline-earth metal cation, and most preferably an alkali metal cation such as sodium, potassium, rubidium or cesium. Additional metal cations such as thallium can be used, and illustrative candidates are disclosed in the specific Examples below.
Modification of the zeolites to incorporate the metal cation can be conducted in conventional fashion, including for example impregnation of the zeolite powder with a solution of an appropriate metal salt followed by drying and calcination, ion exchange into the zeolite powder followed by similar processing or the like. Similarly, the metal cation-loading can take place either before or after the zeolite is bound with a suitable carrier. Typically, the zeolite catalyst will be modified to contain about 1% to about 20% by weight of the metal cation (excluding consideration of any binder present), more typically in the range of about 1% to about 10% by weight.
Preferred zeolite catalysts for use in the invention will have a medium-pore structure. For example, the zeolite catalyst preferably has a constraint index of about 0.5 to about 12, including e.g. zeolite MFI and zeolite beta (BEA). More preferred zeolite catalysts will have a constraint index of about 4 to about 10, with a preferred zeolite catalyst being MFI, having a constraint index of about 8.3. In this regard, this xe2x80x9cconstraint indexxe2x80x9d is a conventional term which is well-known and used in the art to characterize porous catalyst materials including zeolites, and as used herein refers to the value as determined in the conventional fashion which is described for example in Frillette et al, Journal of Catalysis, 67, 218-222 (1981). Advantageous processes of the invention can be carried out using phosphorous-stabilized zeolite catalysts, such as phosphorous-stabilized MFI or phosphorous-stabilized BEA.
The zeolite catalyst will typically be used in bound form with a suitable carrier such as silica, alumina, silica-alumina, or a clay.
As to conditions of the reaction, processes of the invention will typically take place at a temperature of about 200xc2x0 C. to about 500xc2x0 C., more preferably about 350xc2x0 C. to about 450xc2x0 C. The reactions will take place in the vapor phase, preferably conducted continuously by the passage of the reactants into and through a vapor-phase reactor containing the zeolite catalyst. The reactants can be vaporized and combined prior to contacting the zeolite catalyst, or upon or after contacting the zeolite catalyst. The former method is preferable and has been found to provide improved yields. Suitable contact times for processes of the invention are about 0.1 to about 100 seconds, more preferably about 1 to about 20 seconds. Inert gases such as nitrogen can be used to sweep the reacted products from the reaction zone. The catalyst bed can be a fixed or moving catalyst bed (e.g. a fluid bed).
The reaction crude exiting the reaction zone can be collected, and conventional isolation techniques used to recover the products of interest. For example, reaction crudes containing 2- or 4-vinylpyridine can be fractionally distilled to recover the vinylpyridine in a purity exceeding about 95%.
For the purpose of promoting a further understanding of the invention and its principles and advantages, the following specific Examples are provided. It will be understood that these Examples are illustrative, and not limiting, of the invention.